It is generally accepted, that incorporating one or more unconventional refractive optical elements in a multi-element lens or optical system may improve performance, reduce element count, or both.
The unconventional elements most often relied on by optical system and lens designers are elements having an aspheric surface. Many of the earliest computer codes used for optical system and lens design were capable of dealing with aspheric surfaces. For several years now, it has been possible to generate aspheric surfaces, relatively inexpensively, on plastic materials. In recent years, fabrication techniques have been developed which permit such aspheric surfaces to be generated on certain optical glasses.
It is also believed that optical elements made from gradient refractive index (GRIN) transparent materials may be effective in improving optical system performance or reducing element count. However, systems employing the gradient index elements, have, heretofore, remained essentially conceptual. It may be said that the idea of using such elements has been less than enthusiastically endorsed by practitioners of the lens design art.
There may be several reasons for this. Optical design codes capable of dealing with gradient index materials, at any level, have not, until relatively recently, been commercially available. Gradient index materials were originally prepared by applying surface treatments to homogeneous materials. Such treatments include chemical immersion, ion implantation and the like. Such methods of preparation are generally not capable of providing a wide range of index values in a material, or providing a material which is actually thick enough to make a practical lens blank. It is widely believed that GRIN optical elements would be most effective if the refractive index gradient of the glass varied radially, i.e., in a direction perpendicular to the element's optical axis. An axial gradient material, in which refractive index varies with distance along an element's optical axis, is generally believed not to offer any benefit that could not be achieved by an aspheric element.
It is very difficult to produce a radial gradient material on a practically useful scale, and at an acceptable cost. An axial gradient index material, on the other hand, may now be readily produced.
In U.S. Pat. No. 4,929,065 (Hagerty et al.), the disclosure of which is hereby incorporated by reference, a technique is described that allows the fabrication of lens blanks having an index gradient varying in a direction parallel to the optical axis of a lens element. A series of thin glass plates, each of a slightly different refractive index, is stacked together and heated to a predetermined temperature. At this temperature, the plates fuse together, and adjacent plates inter-diffuse. Because of the inter-diffusion of adjacent plates, a transparent medium having a continuously and smoothly varying refractive index is produced. The technique has been developed and refined to the point where glass blanks having refractive index variations greater that about 0.4, for example, from 1.52 to 1.95, may be produced in a range of practical diameter and thickness.
Returning now to optical design aspects, in prior art documents, GRIN elements are generally discussed as part of an optical system including one or more homogenous elements. The GRIN property in such cases is considered as providing no more than an additional degree of freedom for an optical designer. System optimization often involves merely a trial-and-error approach, which, unfortunately, is encouraged by the power, speed and availability of modern computers. Because of this, it is often difficult to determine precisely the exact contribution of a GRIN element to the system performance. It is probable that, by considering GRIN elements in this way, the full potential of GRIN elements is not being realized.
A spherical refractive optical element has all common optical aberrations, to some degree, depending on its actual shape and material. To reduce or eliminate a particular aberration, spherical elements are usually combined in groups of two or more. This has led over the years to well known basic element groups as the Cooke triplet, for correcting third order aberrations; the double Gauss system which uses symmetrical element groups to provide low coma, and thick meniscus elements to reduce astigmatism and field curvature; and several doublet groups for providing low chromatic distortion.
Often, in an optical system involving a group of spherical elements, if the shape of one element is varied to improve a particular aberration in the system, another aberration property of the group is affected. This aberration must be corrected by altering one or more other elements.
It would be useful for optical designers if a GRIN material could be formulated that would allow a single lens element to be produced in a wide variety of shapes while still having at least one selected aberration be essentially zero, or, if not zero, at least some substantially constant value. It would be particularly useful if such a GRIN material could be formulated from an axial GRIN material made by the above referenced technique of Hagerty et al..
This would offer the immediate possibility of using a GRIN element, in a system of optical elements, to correct a selected system aberration, without perturbing another system aberration which had been already adequately corrected. In effect, such a GRIN element could be as standard an optical building-block as any of the well-known conventional multi-element groups are at present.
It is believed that formulation of such GRIN elements may lead to more effective reduction in system element count than is presently believed possible. In certain monochromatic systems, such an element may well be used to replace a system of two or more elements with a single element having merely spherical surfaces.
The present invention is directed to providing such GRIN elements.